Method for surface treating a substrate and device for carrying out the method

ABSTRACT

The invention relates to a method for surface treating a substrate ( 28 ) by means of an electrical corona discharge, wherein aerosols ( 5 ) are sprayed into discharge area defined by an active gap formed between a corona electrode and the substrate ( 28 ) during the discharging, designed such that the aerosols ( 5 ) are sprayed substantially opposite the direction of gravity.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is the U.S. National Stage of International ApplicationNo. PCT/EP2010/068150, filed Nov. 24, 2010, which designated the UnitedStates and has been published as International Publication No. WO2011/064268 and which claims the priorities of German PatentApplications, Serial No. 10 2009 044 638.9, filed Nov. 24, 2009, andSerial No. 10 2010 008 435.0, filed Sep. 2, 2010, pursuant to 35 U.S.C.119(a)-(d).

BACKGROUND OF THE INVENTION

The invention relates to a method for surface treating a substrate and adevice for carrying out the method.

It is known from EP 0 279 371 B1 to apply an aerosol to the surface of aplastic material to attain an improved and more uniform adhesion for asubsequent coating or, with suitable modification of the aerosols, todirectly coat the surface of the plastic material.

In the first case, for example water can be atomized to form aerosols,whereas in the second case, i.e. with direct coating of the plasticsurface, suitable solutions are used.

For producing the corona discharge, two spaced-apart electrodes arerequired, wherein a high voltage is applied to at least one of theelectrodes, while the other electrode is connected to ground. Theaerosols are sprayed into the discharge region, which is defined by aneffective gap formed between a corona electrode and the substrate andhaving a width of about ≦2 mm. The corona treatment itself is known inthe art, for example from the aforementioned EP 0 279 371 B1, so thatexpressively reference is made thereto.

Due to their structure, the atomizers used for producing the aerosolsare not capable to produce aerosols having a uniform size, i.e. aerosolsof different size are produced. It has been observed that thisdiminishes the surface treatment of the substrate, both with respect toproducing adhesion forces, by forming adhesion nests of differentstrength, as well as with respect to applying a coating, when theaerosols are provided for coating the surface of the substrate directly.

It should be noted in this context that the aerosols are atomized fromabove, in the direction of the force of gravity, to the substrate below,as well as the corona discharge itself, by a corresponding arrangementof the spray nozzles associated with the atomizer or the electrodes.

A method for surface treatment of a substrate with an electric coronadischarge is discussed in EP 0 160 889 A2 related to a differenttechnical field, wherein a fluid operates as the corona electrode,wherein the fluid exits from an electrode profile under pressure andadvantageously has a good electrical conductivity, wherein the electrodeprofile is arranged at the bottom with respect to a support electrode.

In contrast to a method in the same technical field, where an effectivegap is formed between the corona electrode and the substrate, theconventional type of fluid electrode has no gap to the substrate, i.e.this electrode contacts the substrate directly.

To obtain a corona discharge, a continuous supply of fluid is requiredwhich should also have good electrical conductivity. I.e., a permanentfluid wall must be placed between the electrode profile from which thefluid exits and the substrate, so as to cause the corona discharge.

However, this method known from the published literature is notpractical for several reasons. For example, parts of the device arepermanently connected to an electric potential, which is not acceptablewith respect to the occupational safety. Although it is recommended toproduce the support electrode from an electrically insulating materialand to electrically insulate the supply line for the fluid from areservoir, this reservoir however must be filled, which islife-threatening during continuous operation of the facility. In thiscase, it is recommended to interrupt the production flow and to switchthe entire device off, which makes operating the device and carrying outthis method uneconomical.

It should also be considered in this context that fluid adhering to thesubstrate cannot be dried off in the desired time. Moreover, the appliedfluid does not completely adhere on the substrate, thus requiringsuitable capture measures for the draining fluid. Nevertheless,contamination of components by the draining fluid cannot be prevented,thus requiring frequent cleaning and possibly damaging these components.

SUMMARY OF THE INVENTION

It is the object of the invention to improve a method of the generictype and a device for carrying out the method such that the quality ofthe coating and/or adhesion of the surface of the substrate aresignificantly improved.

According to one aspect of the invention, this object is solved by amethod for surface treatment of a substrate with an electric coronadischarge, wherein aerosols are sprayed into the discharge regiondefined by an effective gap formed between a corona electrode and thesubstrate during the discharge, wherein the aerosols are sprayedsubstantially against the direction of gravity.

According to another aspect of the invention, this object is solved by adevice having at least two corona electrodes, of which one is connectedto a voltage and the other to ground, and one of the corona electrodesforms a support electrode with a support surface for the substrate andthe other corona electrode forms a counter electrode, wherein anatomizer is provided which can be used to spray the aerosols into thedischarge region of the corona electrodes, wherein the downward orientedsurface of the support electrode forms the support surface.

Surprisingly, it has been observed that the surface treatment of thesubstrate can be significantly improved by spraying the aerosols intothe discharge region from below opposite to the direction of gravity;the improvement is particularly evident in a visually noticeable moreuniform coating.

The larger aerosols may not be transported up to the surface, so thatonly aerosols with insignificant differences in size adhere and/or areeffective. In any event, a significant improvement in the results isobserved, both in the generation of the adhesion forces as well as inthe generation of a coating, whereby coating components are sprayed ontothe surface of the substrate in form of solutions.

With the generation of adhesion forces, the adhesion of the surfacebecomes uniform, which allows subsequent processing, for example acoating with a throughout uniform quality.

This is accompanied by a reduction of rejects and hence significantlyreduced costs, which is significant because the treated substrates aretypically produced in large numbers or large quantities as massproducts.

The novel method can also be realized essentially without additionalcosts and can be carried out compared to the state-of-the-art with adevice that can be adapted without incurring additional costs. I.e., thenovel method is quasi cost-neutral.

Moreover, the method can be used with all suitable substrates, i.e. withplastics, metals, wood or wood materials and the like, also with webs orbodies having stable dimensions, such as plates, profiles and the like.

As already mentioned, various types of fluids can be used as aerosols,both relating to the fluid itself and to the nature of the aerosols.

The device for carrying out the method includes at least two coronaelectrodes, with one of the electrodes being connected to a voltage andthe other to ground, whereby one corona electrode forms a supportelectrode with a support surface in contact with the substrate. Aerosolsare sprayed with an atomizer into the discharge region of the coronaelectrodes formed by an effective gap, thereby subjecting the exposedsubstrate surface facing away from the support electrode to the coronatreatment.

According to the invention, the downward oriented surface of the supportelectrode forms the support surface, so that the aerosols are sprayedwith the atomizer essentially from below upward into the dischargeregion, essentially against the direction of gravity.

Whereas in the state-of-the-art pertaining to the same technical fieldthe discharge region is formed above an imaginary horizontal planeextending through the support electrode, this discharge region accordingto the novel is located below this horizontal plane.

The novel device can basically be implemented by merely arranging theother corona electrode, which associated with and spaced apart from thesupport electrode, as a counter electrode through a rotation by 180°.This eliminates complex structural changes, so that the novel device canbe produced essentially at no extra cost.

The device is preferably housed at least in the region where theaerosols are present in form of a mist and do not adhere to thesubstrate, allowing the components of the aerosols to be reused. Inaddition to economic advantages due to the fact that the aerosolscontain additives which are after recovery once more transported to thetreatment process, the aerosols do not enter the environment due to theencapsulation of the device, thereby effectively preventing a possiblerisk for personnel by inhaling aerosols containing, for example,nanoparticles as an additive.

Additional advantageous embodiments of the invention are recited in thedependent claims.

BRIEF DESCRIPTION OF THE DRAWING

The method according to the invention and the device for carrying outthe method will again be described in the following with reference tothe appended drawings.

It is shown in:

FIG. 1 a device for carrying out the method in a schematic side view;

FIG. 2 a part of the device, also in a schematic side view;

FIG. 3 a partial detail of a substrate produced with the method in across-sectional side view.

FIGS. 1 and 2 show a device for surface treatment of a substrate 4 whichin the illustrated example is formed as a plastic foil to be coated.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The device illustrated in FIG. 1 is for simplification divided intothree sections A, B, C, wherein the sections B and C each show aschematic circuit diagram, for illustrating the method.

The section A is shown again as detail in FIG. 2.

The surface treatment is carried out with an electric corona discharge,with two corona electrodes being provided, wherein one corona electrodeis connected as a support electrode 1 via a line 16 to a generator 9,which produces a high voltage and is connected to a power line 14, andthe other corona electrode is connected via a ground line 15 as acounter electrode 2 to ground. It will be understood that alternativelythe counter electrode 2 may be connected to the voltage and the supportelectrode 1 to ground.

The counter electrode 2 is arranged below the support electrode 1 inrelation to a horizontal plane H extending through the support electrode1, with the substrate 4 contacting a support surface 3 formed on thesupport electrode 1.

Aerosols 5 are sprayed with an atomizer 6 into the discharge regionindicated by the arrows and located between the counter electrode 2 andthe support electrode 1, wherein the nozzles of the atomizer 6 arearranged in the region of the counter electrode 2, so that the aerosolsare introduced essentially against the direction of gravity.

Adhesion forces are produced by the corona discharge on the free surfaceof the substrate 4, as a result of which the substrate surface is thenuniformly coated, for example, when the aerosols contain additives.

If the aerosols are only composed of water, an extraordinarily uniformadhesion of the free surface of the substrate 4 is attained with theinvention, for a subsequent coating, for example application of avarnish, an imprint or the like.

Furthermore, the support electrode 1 constructed as a cylinder, thecounter electrode 2 and the atomizer 6 are arranged in a frame 7, as aresupport rollers 8 across which the substrate 4 is transported, whichextend on both sides of and axis-parallel to the support electrode 1.

In addition, a supply air line 10 and an exhaust air line 11 areinstalled in the discharge region for providing a safe processingenvironment.

The atomizer 6 and the counter electrode 2 are held as an assembly in asupport 12 constructed as a collection container, in which the excessaerosols and/or particles are collected and returned to the closed-loopmaterial flow via a connection 13.

The counter electrode 2 is preferably made of at least one, preferablystranded, wire which can be adjusted by way of guidance and adjustingmeans in relation to the width of the substrate passing through.

The counter electrode 2 requires cleaning during operation formaintaining the quality of the corona discharge and/or the coating withthe aerosols enriched with additives. For this purpose, the counterelectrode 2 is moved through a cleaning device, in particular in anoscillating motion, during the corona discharge. However, such measureis possible only when the counter electrode 2 is connected to ground andthus safe to touch.

While the section A illustrates the material treatment, the supply meansare schematically illustrated in section B, and the process functions insection C.

A supply reservoir 17, in which the materials to be processed foraerosol coating are stored and treated, is associated with the sectionB. After the materials to be recycled have been filtered and cleaned anda throughput measurement 20 is performed, they are supplied to thesupply reservoir 17 via the connection 13.

The coating material is transported continuously from the supplyreservoir 17 into the atomizer 6 via a measurement device 18 for aerosolformation through multi-component atomization 19 via a line 21 and thesupport 12.

Air is supplied with a fan 22 via an air supply 23 to the supply line10, whereas an exhaust duct 25 connected to the exhaust air line 11supplies drawn-in air to an exhaust fan 24.

Lastly, in section C, a control center 26 and data acquisition 27 withprocessing and output for the entire system process are associated asfunctional process components, wherein the functional componentsrelating to the process function C are in operative connection with thecorresponding components of the entire system.

FIG. 3 illustrates a substrate 28, to which a coating 29 was appliedwith the method of the invention, in a significantly enlargedcross-section which is not to scale; the substrate is provided in formof a foil, preferably made of plastic, which has a coating 29 applied onone side with a thickness of ≦200 nm, preferably ≦150 nm.

As can be seen, the coating 29 has a mostly uniform layer thickness,wherein only the surface has a small roughness which, however, isinsignificant for the intended applications.

In principle, a coating 29 may also be applied to the opposite side ofthe substrate 28.

Producing the coating 29 with the novel method results in a coatinghaving a very uniform layer thickness, wherein the additives may haveprepared particles with a diameter of 0.1 nm-1 mm.

Hardening and/or drying can be performed in different ways, depending onthe properties of the applied coating 29. For example, possibilities areheat/air treatment or radiation hardening with electron beam hardening(ESH), UV light, infrared light or with a plasma corona treatment,either in individual treatments or in combinations, which producecross-linking reactions.

Because of the layer thickness of preferably ≦150 nm, a very smallquantity of material may be used for the coating, while simultaneouslyproducing optimized properties due to the uniform layer thickness.

Surprisingly, it has been observed that the quantity of material for theadditives may be reduced significantly, which has proven to beparticularly advantageous because additives are relatively expensivebasic materials.

In addition, the throughput speed of the device with which the coating29 is applied can be increased, so that the production quantity can beincreased significantly.

Moreover, the substantially uniform layer thickness improves theproperties of the coating 29 for particular applications. The quality ofthe coating 29 can also be monitored which was not possible to date tothis extent.

In principle, several layers can be applied, with each layer having athickness according to the invention.

What is claimed is:
 1. A device for surface treatment of a substratewith an electric corona discharge, said device comprising: at least twocorona electrodes, one of the corona electrodes being connected to avoltage and the other one of the corona electrodes being connected toground, with one of the corona electrodes forming a support electrodehaving a downwardly oriented surface to form a support surface for asubstrate, and with the other one of the corona electrodes forming acounter electrode; and an atomizer configured to spray aerosols onlyupwards in a direction opposite to direction of gravity toward thedownwardly oriented surface into a discharge region of the coronaelectrodes, wherein the atomizer and the downwardly oriented surface arepositioned juxtaposed with respect to each other.
 2. The device of claim1, wherein the atomizer is arranged in a region of the counter electrode(2).
 3. The device of claim 1, wherein the support electrode isconstructed as a cylindrical electrode.
 4. The device of claim 1,further comprising support rollers arranged on both sides of the supportelectrode in axis-parallel relationship to the support electrode.
 5. Thedevice of claim 1, wherein the support electrode is connected to thevoltage and the counter electrode is connected to the ground.
 6. Thedevice of claim 1, wherein the counter electrode is constructed as astranded wire electrode.
 7. The device of claim 1, further comprising acleaning device for cleaning the counter electrode.